The present invention relates to steam turbine plants and more particularly to electric power plants operated by steam turbines for which the steam supply is provided by a nuclear boiling water reactor.
In a boiling water nuclear reactor, the nuclear fuel is structured with a suitable geometry to provide for a sustained chain nuclear reaction as the coolant water passes through the fuel arrangement. Conventionally, the nuclear fuel is housed in elongated metallic tubes which are in turn assembled and supported in parallel arrays or bundles. The reactor core is formed from an assembly of the fuel bundles and it is housed in a large pressure vessel with provision for coolant flow along all of the fuel elements. Neutron absorbing control rods are supported within the core for movement relative to the fuel elements.
The design of the core and other reactor parameters determine the reactor power rating. Mechanical, nuclear, hydraulic and other details of the reactor design are the result of development programs aimed at achieving efficient performance for the plant owner.
Since water density is a large determinant of the rate of generation of slow neutrons which are required for the controlled propagation of the chain nuclear reaction, the power operating level of the reactor is partly determined by the accumulation of steam voids in the core volume. Increased coolant flow causes faster fuel rod cooling with reduced boiling and, accordingly, reduced void accumulation and higher reactor power. Decreased coolant flow has the opposite effects. Typically, coolant flow control can be used to control the boiling water reactor power level within a range of about 20% or 25% with a preset control rod placement.
The reactor generated steam is normally directed through separators and dryers within the pressure vessel, and the dry saturated steam is directly channeled at a pressure such as 1000 psi and a temperature such as 545.degree.F to the utilization equipment, i.e., the turbine generator unit(s) of the electric power plant. Separated water is combined in the pressure vessel with external and internal recirculation flows and with return and makeup feedwater flow.
Since the boiling water reactor plant is the direct cycle type and since outlet steam pressure and reactor vessel pressure affect the void accumulation in the reactor core, it is desirable to operate the turbine inlet valves to determine the turbine and generator load level subject to pressure regulating demands of the reactor. With reactor pressure maintenance within a relatively narrow pressure band such as about 30 psi, reactor power level is controlled by coolant flow control within a limited range or by control rod movement if a different power range is required to meet load demand on the turbine generator unit(s).
In general, the steam turbine energization level is determined by the flow of the turbine inlet steam which in turn is determined by the steam conditions at the outlet of the steam source and by steam inlet valve positioning. The turbine drive power supplied for the plant generator(s) is desirably controlled to satisfy electrical load demand and frequency participation demand placed on the electric power plant by the plant operator or by an economic dispatch computer or by other means.
At substantially constant temperature throttle steam, turbine power is proportional to turbine steam flow, and if the throttle pressure is also substantially constant, the steam flow is proportional to impulse chamber steam pressure or the ratio of the impulse chamber steam pressure to the throttle steam pressure. As already indicated, positioning of the inlet steam valving must provide for reactor vessel pressure regulation as well as turbine energization level control. When the boiling water reactor power level corresponds to the plant load demand, the turbine inlet valves are positioned to produce both the desired reactor vessel pressure and the turbine steam flow required for satisfying plant electrical load demand.
A steam bypass system is also usually provided to direct steam flow from the reactor outlet to the plant condenser under certain conditions. Steam bypass in effect provides an interface between the boiling water reactor and the steam turbine during reactor startup and shutdown and during other periods such as during load rejection. In these cases, steam supplied by the reactor but not needed by the turbine is channeled to the condenser under control imposed on the bypass system by the throttle pressure control system.
Presently, boiling water reactor-steam turbine systems include a pair of redundant analog pressure controllers. These pressure controllers are typically conventional operational amplifiers having input circuitry that performs a summing function with respect to applied input signals; and are provided with a proportional characterization in accordance with a lead/lag function, for example. A bias is manually applied to one of the two parallel connected controllers so that the output signal of one is higher than the output signal of the other by the amount of the applied bias. The output signals of the controller are connected to a high signal selector, which passes only the higher of the two signals to control the steam throttle pressure. Thus, only one of the controllers is effectively controlling the system at any one time. Shifting the bias, of course, renders the output signal of the other controller effective to control the valves. Should one of the controllers become defective, the other controller can be manually placed in control of the system. However, if such malfunction results in the output signal being higher than normal, the defective controller would still be governing the system.
It is desirable, for such a pressure controller organization, to be so structured that regardless of the type of failure, the operation of either one of the controllers is transferred automatically to the other nondefective controller. Also, in the event of a malfunction of any of the components in both of the controllers, it is desirable that the valves be automatically tranferred to manual operation. With the benefit of such a system, reliable control of the boiling water reactor turbine control system can be effected; and the throttle pressure control can be automatically transferred to manual control of the bypass and governor valves immediately upon the failure of the automatic pressure control portion of the system. Not only is the immediate detection of pressure controller failures necessary for safety in the system, but it is also desirable that any disturbances to the system resulting from the opening or closing of valves due to such malfunctions be avoided.